Reference picture lists modification

ABSTRACT

Systems, methods, and instrumentalities are disclosed relating to modifications to reference picture lists used for multiple layer video coding. A bitstream that may include a reference picture list of a slice may be received. An indication to reposition a reference picture within the reference picture list from a first position to a second position may be received. An indication to insert a reference picture within the reference picture list at a position may be received. The reference picture may be repositioned and/or inserted in the reference picture list in response to receiving the indication. A reference picture previously associated with the position may be shifted in the reference picture list according to the indication to reposition and/or insert the reference picture, although an indication to reposition the reference picture previously associated with the position may not be received. The slice may be decoded using the reference picture list.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/698,224, filed Sep. 7, 2012, U.S. Provisional PatentApplication No. 61/732,021, filed Nov. 30, 2012, and U.S. ProvisionalPatent Application No. 61/762,520, filed Feb. 8, 2013, the contents ofwhich are hereby incorporated by reference herein.

BACKGROUND

Video coding systems may be used to compress digital video signals toreduce the storage need and/or transmission bandwidth of such signals.Among the types of video coding systems, such as block-based,wavelet-based, and object-based systems, block-based hybrid video codingsystems may be widely used and deployed. Examples of block-based videocoding systems include, but are not limited to those described ininternational video coding standards such as the MPEG1/2/4 part 2,H.264/MPEG-4 part 10 AVC and VC-1 standards.

SUMMARY

Systems, methods, and instrumentalities are disclosed relating tomodifications to reference picture lists used for multiple layer videocoding. A device, such as a decoder, for example, may perform thefollowing. A bitstream may be received. The bitstream may be a scalablebitstream. The bitstream may include one or more reference picture listsof a slice (e.g., a P slice, a B slice, or the like) of the bitstream. Areference picture list may include a plurality of reference pictures.The reference pictures may include temporal reference pictures and/orinter-layer reference pictures for the slice. The reference picture listmay include temporal reference pictures positioned before inter-layerreference pictures.

An indication may be received. The indication to reposition thereference picture may be a variable (e.g., a flag) in a slice header ofthe slice. The indication may indicate that a reference picture of theplurality of reference pictures may be repositioned within the referencepicture list from a first position to a second position. The firstposition may be before or after the second position in the referencepicture list.

The reference picture may be repositioned within the reference picturelist from the first position to the second position, for example, inresponse to receiving the indication. A reference picture previouslyassociated with the second position may be shifted in the referencepicture list according to the indication to reposition the referencepicture. An indication to reposition the reference picture previouslyassociated with the second position may not be received. The referencepicture may be an inter-layer reference picture. The inter-layerreference picture may be repositioned in front of a temporal referencepicture within the reference picture list. The slice may be decodedusing the reference picture list.

The reference picture previously associated with the second position andreference pictures previously associated with positions between thefirst position and the second position may be shifted in the referencepicture list according to the indication to reposition the referencepicture, for example, if there are one or more reference picturesbetween the first position and the second position. Indications may notbe received for the reference picture previously associated with thesecond position or the reference pictures previously associated withpositions between the first position and the second position.

A sub-set of the plurality of reference pictures within the referencepicture list may be repositioned according to the indication toreposition the reference picture. The sub-set of the plurality ofreference pictures may include the reference picture and the referencepicture may be repositioned from the first position to the secondposition. The remaining reference pictures of the sub-set of referencepictures may be repositioned to positions after the second position. Oneor more reference pictures not associated with the sub-set of theplurality of reference pictures may be shifted within the referencepicture list according to the indication to repositioning the referencepicture. The sub-set of the plurality of reference pictures may beinter-layer reference pictures. The inter-layer reference pictures maybe repositioned in front of a temporal reference picture within thereference picture list, for example, without receiving an indication toreposition the temporal reference picture.

A bitstream may be received. The bitstream may be a scalable bitstream.The bitstream may include one or more reference picture lists and/or oneor more reference picture sets. A reference picture list may include aplurality of temporal reference pictures, for example, for temporalprediction of a slice (e.g., a P slice, a B slice, or the like). Areference picture set may include a plurality of inter-layer referencepictures, for example, for inter-layer prediction of the slice.

An indication to insert a reference picture of the reference picture setat a position within a reference picture list may be received. Thereference picture may be inserted into the reference picture list at theposition. A reference picture previously associated with the positionmay be shifted in the reference picture list according to the indicationto insert the reference picture. An indication to reposition thereference picture previously associated with the position of thereference picture list may not be received. The reference picture may bean inter-layer reference picture. The reference picture previouslyassociated with the position of the reference picture list may be atemporal reference picture. A size of the reference picture list mayincrease upon the insertion of the reference picture at the positionwithin the reference picture list. The slice of the bitstream may bedecoded using the reference picture list.

A plurality of reference pictures from the reference picture set may beinserted into the reference picture list in accordance with theindication to insert the reference picture. The plurality of referencepictures may include the reference picture. The reference picture may beinserted at the position within the reference picture list, and forexample, the remaining reference pictures of the plurality of referencepictures may be included after the position. The plurality of referencepictures from the reference picture set may be inter-layer referencepictures.

A device (e.g., an encoder) may perform a first encoding of a bitstreamusing a first reference picture list and collect encoding statisticswhile performing the first encoding. The device may reorder one or morereference pictures of the reference picture list based on the collectedencoding statistics to create a second reference picture list. Thedevice may perform a second encoding the bitstream using the secondreference picture list. The reordering of the one or more referencepictures may be based on usage of the one or more reference picture inthe reference picture list during the first encoding. The reordering ofthe one or more reference pictures may be based on motion-compensateddistortion between an original video block of the video stream and oneor more prediction blocks of the bitstream. The device may transmit thebitstream with the second reference picture list to another device(e.g., a decoder).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a block-based videoencoder.

FIG. 2 is a diagram illustrating an example of a block-based videodecoder.

FIG. 3 is a diagram illustrating an example of a two view video codingstructure.

FIG. 4 is a diagram illustrating an example of the reference picturelist construction using temporal and inter-layer reference pictures in adecoded picture buffer (DPB).

FIG. 5 is a diagram illustrating an example of list modificationsignaling.

FIG. 6 is a flowchart illustrating an example of a reference picturelist modification decoding process.

FIG. 7 is a diagram illustrating an example of list modificationsignaling.

FIG. 8 is a flowchart illustrating an example of an encoder using areference picture reordering process.

FIG. 9A is a diagram illustrating an example of a reference picture listconstruction with a single indicator.

FIG. 9B is a diagram illustrating an example of a reference picture listconstruction with a single indicator.

FIG. 10A is a system diagram of an example communications system inwhich one or more disclosed embodiments may be implemented.

FIG. 10B is a system diagram of an example wireless transmit/receiveunit (WTRU) that may be used within the communications systemillustrated in FIG. 10A.

FIG. 10C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 10A.

FIG. 10D is a system diagram of another example radio access network andanother example core network that may be used within the communicationssystem illustrated in FIG. 10A.

FIG. 10E is a system diagram of another example radio access network andanother example core network that may be used within the communicationssystem illustrated in FIG. 10A.

DETAILED DESCRIPTION

An inter-layer may include a processed base layer and/or an upsampledbase layer. For example, an inter-layer, a processed base layer, and/oran upsampled base layer may be used interchangeably. An inter-layerreference picture, a processed base layer reference picture, and/or anupsampled base layer reference picture may be used interchangeably. Aninter-layer video block, a processed base layer video block, and/or anupsampled base layer video block may be used interchangeably. There maybe a temporal relationship between an enhancement layer, an inter-layer,and a base layer. For example, a video block and/or a picture of anenhancement layer may be associated with a temporally correspondingvideo block and/or picture of the inter-layer and/or the base layer.

A slice may include a picture and/or a video block. A video block may bean operational unit at any tier and/or level of the bitstream. Forexample, a video block may refer to an operational unit at the picturelevel, the block level, the slice level, etc. A video block may be ofany size. For example, a video block may refer to a video block of anysize, such as a 4×4 video block, an 8×8 video block, a 16×16 videoblock, or the like. For example, a video block may refer to a predictionunit (PU), a smallest PU (SPU), or the like. A PU may be the video blockunit used for carrying the information related to motion prediction, forexample, including a reference picture index and MV. One PU may includeone or more smallest PUs (SPUs). Although SPUs in the same PU may referto the same reference picture with identical MVs, storing motioninformation in units of the SPUs may facilitate motion informationretrieval in some implementations. Motion information (e.g., a MV field)may be stored in units of the video block, such as the PU, the SPU, orthe like. Although examples described herein may be described withreference to pictures, slices, video blocks and/or PUs, any operationalunit of any size (e.g., a picture, a video block, a PU, a SPU, or thelike) may be used.

FIG. 1 is a diagram illustrating an example of a block-based hybridvideo encoding system. The input video signal 102 may be processed blockby block. A video block unit may include 16×16 pixels. Such a block unitmay be referred to as a macroblock (MB). In High Efficiency Video Coding(HEVC), extended block sizes (e.g., which may be referred to as a“coding unit” or CU) may be used to efficiently compress high resolution(e.g., 1080p and beyond) video signals. In HEVC, a CU may be up to 64×64pixels. A CU may be partitioned into prediction units (PUs), for whichseparate prediction methods may be applied.

For an input video block 102 (e.g., a MB or a CU), spatial prediction160 and/or temporal prediction 162 may be performed. Spatial prediction(e.g., intra prediction) may use pixels from already coded neighboringblocks in the same video picture/slice to predict the current videoblock. Spatial prediction may reduce spatial redundancy inherent in thevideo signal. Temporal prediction (e.g., inter prediction or motioncompensated prediction) may use pixels from already coded video pictures(e.g., which may be referred to as reference pictures) to predict thecurrent video block. Temporal prediction may reduce temporal redundancyinherent in the video signal. A temporal prediction signal for a givenvideo block may be signaled by one or more motion vectors. A motionvector may indicate the amount and/or the direction of motion betweenthe current block and its prediction block in the reference picture.

A reference picture index may be provided for a video block. Forexample, if multiple reference pictures are supported (e.g., as may bethe case for H.264/AVC and/or HEVC), then for a video block, itsreference picture index may be sent. The reference index may be used toidentify from which reference picture in the reference picture store 164(e.g., which may be referred to as a decoded picture buffer (DPB)) thetemporal prediction signal comes. The mode decision block 180 in theencoder may select a prediction mode, for example, after spatial and/ortemporal prediction.

The prediction block may be subtracted from the current video block 116.The prediction residual may be transformed 104 and/or quantized 106. Thequantized residual coefficients may be inverse quantized 110 and/orinverse transformed 112 to form the reconstructed residual. Thereconstructed residual may be added to the prediction block 126 to forma reconstructed video block. In-loop filtering such as, but not limitedto a deblocking filter, a Sample Adaptive Offset, and/or Adaptive LoopFilters may be applied by loop filter 166 on the reconstructed videoblock before it is put in the reference picture store 164 and/or used tocode future video blocks. To form the output video bitstream 120, acoding mode (e.g., inter coding or intra coding), prediction modeinformation, motion information, and/or quantized residual coefficientsmay be sent to the entropy coding unit 108 to be compressed and packedto form the bitstream.

FIG. 2 is a block diagram illustrating an example of a block-based videodecoder. The video bitstream 202 may be unpacked and entropy decoded atentropy decoding unit 208. The coding mode and prediction informationmay be sent to the spatial prediction unit 260 (e.g., if intra coded)and/or the temporal prediction unit 262 (e.g., if inter coded) to formthe prediction block. If inter coded, the prediction information maycomprise prediction block sizes, one or more motion vectors (e.g., whichmay indicate direction and amount of motion), and/or one or morereference indices (e.g., which may indicate from which reference picturethe prediction signal is to be obtained). Motion compensated predictionmay be applied by the temporal prediction unit 262 to form the temporalprediction block. The residual transform coefficients may be sent toinverse quantization unit 210 and inverse transform unit 212 toreconstruct the residual block. The prediction block and the residualblock may be added together at 226. The reconstructed block may gothrough in-loop filtering before it is stored in reference picture store264. The reconstructed video in reference picture store may be sent outto drive a display device, as well as used to predict future videoblocks.

A single layer video encoder may take a single video sequence input andgenerate a single compressed bit stream transmitted to the single layerdecoder. A video codec may be designed for digital video services (e.g.,such as but not limited to sending TV signals over satellite, cable,and/or terrestrial transmission channels). With video centricapplications deployed in heterogeneous environments, multi-layer videocoding technologies may be developed as an extension of the video codingstandards to enable various applications. For example, scalable videocoding technologies may be designed to handle more than one video layer,where a layer (e.g., each layer) may be decoded to reconstruct a videosignal of a particular spatial resolution, temporal resolution,fidelity, and/or view.

FIG. 3 is a diagram illustrating an example of a two view video codingstructure. FIG. 3 illustrates an example of temporal andinter-dimension/layer prediction for two-view video coding. Inter-layerprediction (e.g., illustrated by dashed lines) may be used to improvethe compression efficiency by exploring the correlation among multiplevideo layers. For example, the inter-layer prediction may be performedbetween two views.

Signaling and/or construction of one or more reference pictures used forinter-layer prediction and/or temporal prediction may be describedherein. Signaling and/or construction of a current reference picturelist (e.g., in HEVC) may be described herein. Signaling related toreference picture list construction and/or modification for mixedtemporal and inter-layer prediction may be described herein. The terms“temporal prediction,” “motion prediction,” “motion compensatedprediction,” and “inter prediction” may be used interchangeably herein.The terms “inter-view” and “inter-layer” may be used interchangeablyherein. The terms “reference picture store” and “decoded picture buffer”or “DPB” may be used interchangeably herein.

A reference picture list may include one or more reference pictures thatmay be used for temporal motion compensated prediction of a P and/or Bslice (e.g., in HEVC). For the decoding process of a P slice, there maybe one reference picture list (e.g., list0). For the decoding process ofa B slice, there may be two reference picture lists (e.g., list0 andlist 1). A default reference picture list may be constructed based onthe reference picture set (RPS) and/or the distance between thereference pictures and the current coding picture (e.g., in HEVC WD9).In case the reference pictures are not in the default order in the list,the list may be modified (e.g., using the list modification specified inHEVC) to assign a reference picture (e.g., each reference picture) intothe list. Table 1 is an example of a reference picture list modificationsyntax (e.g., the syntax specified in HEVC WD9). The HEVC listmodification may simplify the list construction process and may allowflexibility in the use of reference picture lists L0 and L1 for temporalmotion compensation.

TABLE 1 Example of a Reference Picture List Modification SyntaxRef_pic_list_modification( ) { Descriptor   if( slice_type = = P ||slice_type = = B ) {     ref_pic_list_modification_flag_l0 u(1)     if(ref_pic_list_modification_flag_l0 && NumPocTotalCurr > 1 )       for( i= 0; i <= num_ref_idx_l0_active_minus1; i++ )         list_entry_l0[ i ]u(v)   }   if( slice_type = = B ) {    ref_pic_list_modification_flag_l1 u(1)     if(ref_pic_list_modification_flag_l1 && NumPocTotalCurr > 1 )       for( i= 0; i <= num_ref_idx_l1_active_minus1; i++ )         list_entry_l1[ i ]u(v)   } }

The reference picture list may include one or more temporal (e.g., shownby solid lines) and/or inter-layer (e.g., shown by dashed lines)reference pictures as shown in FIG. 3, for example, for multiple layervideo coding. An inter-layer reference picture may be derived from aninter-layer reference picture set and/or inferred from a co-locatedreference picture's RPS from its dependent layer. The default referencepicture list may be constructed by putting temporal reference picturesin front of the inter-layer reference pictures in the reference picturelist. This may be done because there may be a high correlation in thetemporal domain. The inter-layer reference picture may present highercorrelation than a temporal reference picture for certain scenarios. Forexample, when the temporal distance from the temporal reference picturesis relatively large, inter-layer reference pictures may offer highercorrelation than temporal references. A multi-pass encoder may be ableto adjust the position of inter-layer reference picture based on thestatistics collected from the first pass encoding, for example, toimprove the coding efficiency. Appropriate reference picture listmodification may be designed to handle reordering of a temporalreference picture and/or an inter-layer reference picture, for example,for a scalable video system.

An inter-layer reference picture may be placed behind one or moretemporal reference pictures. A modification implementation (e.g., a HEVClist modification method) may be used to signal a reference picture inthe list, for example, to rearrange the position of the inter-layerreference picture. FIG. 4 is a diagram illustrating an example of thereference picture list construction (e.g., using list 0 or L0 as anexample) using temporal reference picture and inter-layer referencepictures in the DPB. For example, for the current coding picturePOC_(1,3), there may be four temporal reference pictures (e.g.,POC_(1,1), POC_(1,2), POC_(1,4), POC_(1,5)), one co-located inter-layerreference picture (POC_(0,3)), and four non-colocated inter-layerreference pictures (e.g., POC_(0,1), POC_(0,2), POC_(0,4), POC_(0,5)).The default reference picture list 0 (L0) may be constructed by placingthe temporal reference pictures prior to the inter-layer referencepictures. In case the co-located reference picture (POC_(0,3)) may bethe best reference picture, the co-located reference picture may bemoved to the first position of L0. The reference position index of eachreference picture may be signaled (e.g., list_entry_10) to construct thereordered L0. For example, as shown in FIG. 4, nine syntax elements maybe signaled to the decoder even though the position of one referencepicture (e.g., the collocated inter-layer reference POC_(0,3)) may bechanged.

Reference picture list modification signaling to support mixed temporaland inter-layer reference picture reordering may be described herein.Reordering of a combined list of temporal and inter-layer referencepictures may be described herein. Reference picture list modificationmay be applied on the slice level. The reference picture list(s) mayremain unchanged for the video blocks in the slice.

The position of a reference picture in a reference picture list may besignaled when the reference picture list modification flag is set, forexample, as shown in Table 1 and/or FIG. 4. Reference picture listmodification signaling may signal a reference picture (e.g., only thereference picture) whose order may be changed from the default list, forexample, to save overhead bits. An indicator of the reordered picturemay be added into a reference picture list modification syntax. Table 2illustrates an example of reference picture list modification signaling.

TABLE 2 Example of a Reference Picture List Modification Syntaxref_pic_list_modification( ) { Descriptor if( slice_type != 2) { // Pslice or B slice   ref_pic_list_modification_flag_l0 u(1)   if(ref_pic_list_modification_flag_l0 && NumPocTotalCurr > 1)   num_ref_pic_reordered_l0_minus1 ue(v)    for ( i =0; i <num_ref_pic_reordered; i++ ) {       init_list0_idx u(v)      modified_list0_idx u(v)     }   }   if( slice_type = = 1) { // Bslice     ref_pic_list_modification_flag_l1 u(1)     if(ref_pic_list_modification_flag_l1 && NumPocTotalCurr > 1 )      num_ref_pic_reordered_l1_minus1 ue(v)       for ( i =0; i <num_ref_pic_reordered; i++ ) {        init_list1_idx u(v)       modified_list1_idx u(v)       }   } }

The syntax element init_listX_idx and modified_listX_idx (e.g., X may be0 or 1) may indicate a change from the initial reference picture list tothe reference picture list to be used for decoding a video block (e.g.,a slice). The syntax element num_ref_pic_reordered_1X_minus1 plus 1 mayindicate a number of reference pictures to be reordered in the referencepicture list. The syntax element init_listX_idx may indicate an index ofa reordered picture in the initial listX (e.g., as specified byRefPicSetCurrTempListX in HEVC). The syntax element modified_listX_idxmay indicate an index of a reordered picture to be placed in a modifiedreference picture list LX.

Signaling may indicate (e.g., only indicate) those reference pictureswhose order is to be changed in the reference picture list. A referencepicture whose ordering is not being changed may be the same as in theinitial reference picture list even though its position index may bechanged. For example, a reference picture whose ordering is not beingdirectly changed (e.g., only indirectly changed via the reordering ofthe reference picture indicated by the signaling) may be the same as inthe initial reference picture list even though its position index may bechanged. Reference picture list modification may be applied on the slicelevel. The reference picture list(s) may remain unchanged for the videoblocks in the slice.

FIG. 5 is a diagram illustrating an example of list modificationsignaling. For example, three syntax elements may be used in the exampleof FIG. 5 instead of the nine syntax elements used in the example ofFIG. 4. The signaling may be applied for single layer video codingsystems and/or multi-layer video coding systems. For example, areference picture (e.g., POC_(0,3)) may be in a first position (e.g.,position four) in a reference picture list. An indication (e.g.,num_ref_pic_reordered_1X_minus1) may be received to indicate a number ofreference pictures to be reordered in the reference picture list. Anindication (e.g., init_listX_idx) may be received to indicate that areference picture (e.g., POC_(0,3)) in the reference picture list may berepositioned. An indication (e.g., modified_listX_idx) may be receivedto indicate a position (e.g., position zero) for the reference pictureafter it is repositioned. The reference picture (e.g., POC_(0,3)) may berepositioned from a first position (e.g., position four) to a secondposition (e.g., position zero) based on the indication (e.g.,num_ref_pic_reordered_1X_minus1, init_listX_idx, and/ormodified_listX_idx). The reference pictures previously associated withthe second position (e.g., position zero) and those positions betweenthe first position and the second position (e.g., positions one, two,and three) may be shifted based on the repositioning of the referencepicture (e.g., POC_(0,3)). For example, the reference picturespreviously associated with the second position (e.g., position zero) andthose positions between the first position and the second position(e.g., positions one, two, and three) may be shifted without receivingan indication to shift those reference pictures. Although the referencepicture (e.g., POC_(0,3)) is repositioned to a position before it in thereference picture list, the signaling may be used to reposition thereference picture to a position after it in the reference picture list.Although one reference picture is repositioned based on the signaling, aplurality of reference pictures may be repositioned based on thesignaling, for example, as described herein.

A reference picture list initialization process for temporal predictionand/or for inter-layer prediction may be provided. For example, thesyntax element RefPicListTempX may indicate an initial reference picturelist. The syntax element cIdx may indicate the position index of thereference picture(s) in the RefPicListTempX (e.g., X being 0 or 1). Theinput to reference picture list modification may be one or more of theinitial reference picture list (e.g., RefPicListTempX), the initialindex of reference picture (e.g., cIdx), the number of referencepicture(s) reordered (e.g., num_ref_pic_reordered_1X_minus1), the indexof the reference picture in RefPicListTempX (e.g., init_listX_idx), atotal number of reference pictures used by current picture in DPB (e.g.,NumPocTotalCurr), and the index of a reordered reference picture in themodified reference picture list (e.g., modified_listX_idx). The outputof the reference picture list modification may be the reference picturelist RefPicModifiedListX.

When decoding a P slice and/or a B slice, there may be one or morereference picture(s) in RefPicTempListX (e.g., NumPocTotalCurr>1). Thefollowing procedure may be conducted to construct a modified referencepicture list (e.g., RefPicModifiedListX):

  cIdx = 0   i = 0;   for( rIdx=0; rIdx <= num_ref_idx_lX_active_minus1;rIdx++) {     if ( (i <= num_ref_pic_reordered_lX_minus1) && (rIdx ==modified_listX_idx[i])) {       RefPicModifiedListX[rIdx] =RefPicTempListX[init_listX_idx[i]];       i ++;     }     else {      is_ref_pic_reordered = true;       while ((is_ref_pic_reordered ==true) && (cIdx < NumPocTotalCurr)) {       is_ref_pic_reoredered =false;         for (j = 0; j <= num_ref_pic_reordered_lX_minux1; j ++) {         if (cIdx == init_listX_idx[j]) {           cIdx ++;          is_ref_pic_reordered = true;           break;          }        }       }       if (cIdx < NumPocTotalCurr)        RefPicModifiedListX[rIdx] = RefPicTempListX[cIdx++];     }   }

FIG. 6 is a flowchart illustrating an example of reference picture listmodification decoding. Reference picture list modification signallingand/or decoding may be applied to multiple layer video coding and/or tosingle layer video coding with temporal reference pictures.

An indicator (e.g., modified_listX_idx) may be signaled in a bitstreamto specify a position within a reference picture list. A subset of theinitial listX (e.g., as specified by RefPicSetCurrTempListX) may berepositioned, for example, repositioned to the position indicated by theindicator. The subset of the initial listX may be inter-view referencepictures and/or inter-layer reference pictures. The indicator may be avariable, for example, a single indicator (e.g., modified_listX_idx).The encoder and/or decoder may be aware (e.g., mutually aware) that theplacement of the reference pictures are designated for a subset ofreference pictures (e.g., inter-layer reference pictures). The indicatormay be a variable. The indicator may be signaled in a parameter set(e.g., Video Parameter Set (VPS), Sequence Parameter Set (SPS), PictureParameter Set (PPS), or the like). The indicator may be signaled in aslice header (e.g., a flag in the slice header) for reference picturelist construction. FIG. 9A illustrates an example reference picture listconstruction with a single indicator signaling.

The following may be conducted to construct a reference picture list(e.g., RefPicListX) with a single indicator (e.g., modified_listX_idx):

cIdx = 0 for( rIdx=0; rIdx <= num_ref_idx_lX_active_minus1; rIdx++) {   if ( rIdx == modified_listX_idx ) {     i = rpsSubsetIdx;     j =num_ref_pic_of_rpsSubset;   while ((j) && (rIdx <=num_ref_idx_lX_active_minus1))      {       RefPicListX[rIdx++] =RefPicTempListX[i++];       j −−;      }    }    else {       if (cIdx== rpsSubsetIdx) {        cIdx += num_ref_pic_of_rpsSubset;        }      RefPicListX[rIdx++] = RefPicTempListX[cIdx++];    } }

For example, rpsSubsetIdx may be the index of the first picture of thereference picture subset of the initial listX, for example, as specifiedby RefPicSetCurrTempListX. For example, num_ref_pic_of_rpsSubset may bethe number of reference pictures that may belong to the designatedreference picture subset.

A sub-set of the plurality of reference pictures within the referencepicture list may be repositioned according to an indication toreposition a reference picture of the sub-set, for example, as shown inFIG. 9A. An indicator (e.g., num_ref_pic_of_rpsSubset) may be receivedthat indicates the sub-set of reference pictures that are to berepositioned within the reference picture list. An indicator (e.g.,rpsSubsetIdx) may be included in the slice header of a reference picture(e.g., POC_(0,3)). The reference picture may be included in the sub-setof reference pictures. The indicator may indicate a position (e.g.,position one) within in the reference picture list to reposition thereference picture (e.g., POC_(0,3)). The reference picture may berepositioned to the position within the reference picture list based onthe indication. The remaining reference pictures in the sub-set ofreference pictures (e.g., POC_(0,2), POC_(0,1), POC_(0,4), andPOC_(0,5)) may be repositioned in the reference picture list after thereference picture indicated by the indicator (e.g., to positions two,three, four, and five). One or more reference pictures not associatedwith the sub-set of the plurality of reference pictures (e.g.,POC_(1,1), POC_(1,4), and POC_(1,5)) may be shifted within the referencepicture list according to the repositioning of the sub-set of referencepictures (e.g., to positions six, seven, and eight), for example,although indicators to reposition the reference pictures were notreceived. The sub-set of the plurality of reference pictures may beinter-layer reference pictures and/or inter-view reference pictures. Theinter-layer reference pictures may be repositioned in front of atemporal reference picture within the reference picture list, forexample, without receiving an indication to reposition the temporalreference picture.

A reference picture list modification implementation may be utilized fora multi-layer video coding system. FIG. 7 is a diagram illustrating anexample of reference picture list modification signaling. For example,there may be two reference picture sets, a temporal reference pictureset and an inter-layer reference picture set. Reference picture listmodification may be applied on the slice level. The reference picturelist(s) may remain unchanged for the video blocks in the slice. FIG. 7may illustrate one collocated reference picture in the inter-layerreference picture set, although more than one collocated referencepicture may be utilized. The collocated inter-layer reference picture(e.g., POC_(0,3)) may be used the most as it may be captured at the sametime instance as the current picture being coded (e.g., POC_(1,3)). Thecollocated inter-layer reference picture may offer higher correlationthan temporal reference pictures.

A modification of the temporal reference picture list may be compliantwith the list modification process utilized in a single layer videocoding standard (e.g., HEVC or AVC). Inter layer reference picture listmodification may be deployed on the top of a modified temporal referencepicture list to form the final reference picture list for multi-layervideo coding. The placement of inter-layer reference pictures in thereference picture lists may be specified.

An indicator (e.g., the inter_layer_ref_pic_modification_flag) may beincluded in a slice header to indicate whether the modification of theinter-layer reference picture is performed. The indicator may be avariable (e.g., a flag or the like) in the slice header of the currentpicture. The inter-layer reference picture may be positioned aheadand/or behind a temporal reference picture in the reference picturelist.

If the indicator (e.g., inter_layer_ref_pic_modification_flag) is set to1, then additional syntax element pairs may be signaled to indicate theplacement of the inter-layer reference picture in the reference picturelist. One or more syntax elements (e.g., indicators) may be signaled foran inter-layer reference picture to be inserted in the reference picturelist. A syntax element specifying how many pairs exist may be signaled.

A syntax element in the pair (e.g., inter_layer_ref_pic_idx) may be usedto indicate which one of the inter-layer reference pictures from theinter-layer reference picture set may be added into the referencepicture list. A syntax element in the pair (e.g., modified_listX_idx)may be used to indicate the index in the reference picture list LXassigned to the inter-layer reference picture represented byinter_layer_ref_pic_idx. As an inter-layer reference picture is insertedinto the reference picture list, the following reference pictures may beshifted (e.g., to the right and/or to the back) in the reference picturelist (e.g., by 1).

A reference picture (e.g., an inter-layer reference picture) may beinserted into a reference picture list for a slice, for example,according the signaling described in reference to FIG. 7. A bitstream(e.g., a scalable bitstream) may include a reference picture list and/ora reference picture set. The reference picture list may include aplurality of temporal reference pictures, for example, for temporalprediction of the slice. The reference picture set may include aplurality of inter-layer reference pictures for the slice, for example,for inter-prediction of the video block.

An indication (e.g., inter_layer_ref_pic_modification_flag) to insert aninter-layer reference picture (e.g., POC_(0,3)) of the reference pictureset at a position within the reference picture list may be received. Anindication (e.g., inter_layer_ref_pic_idx) may indicate the referencepicture (e.g., POC_(0,3)) to be inserted into the reference picturelist. An indication (e.g., modified_listX_idx) may indicate the position(e.g., position two) where the reference picture (e.g., POC_(0,3)) is tobe inserted. The inter-layer reference picture (e.g., POC_(0,3)) may beinserted into the reference picture list at the position. A temporalreference picture previously associated with the position (e.g.,POC_(1,4)) may be shifted in the reference picture list, for example,without receiving an indication to reposition the temporal referencepicture. For example, one or more temporal reference pictures (e.g.,POC_(1,4) and POC_(1,5)) may be repositioned according to the indicationto insert the inter-layer reference picture (e.g., POC_(0,3)). The sizeof the reference picture list may increase upon the insertion of theinter-layer reference picture, for example, from four reference picturesto five reference pictures).

Table 3 illustrates an example syntax structure.

TABLE 3 Example of a Proposed Reference Picture List Modification Syntaxinter_layer_ref_pic_list( ) { Descriptor   if( slice_type != 2 &&NumInterLayerRefPic > 1) { // P slice or B slice    inter_layer_ref_pic_modification_flag_l0 u(1)     if(inter_layer_ref_pic_modification_flag_l0 )      num_mod_inter_layer_ref_pic_l0_minus1 ue(v)       for ( i =0; i <=num_mod_inter_layer_ref_pic_l0_minus1; i++ ) {        inter_layer_ref_pic_idx[i] u(v)         modified_list0_idx[i]u(v)       }   }   if( slice_type = = 1 && NumInterLayerRefPic > 1) { //B slice     inter_layer_ref_pic_modification_flag_l1 u(1)     if(inter_layer_ref_pic_modification_flag_l1 )      num_mod_inter_layer_ref_pic_l1_minus1 ue(v)       for ( i =0; i <num_mod_inter_layer_ref_pic_l1_minus1; i++ ) {        inter_layer_ref_pic_idx[i] u(v)         modified_list1_idx[i]u(v)       }   } }

The following pseudo-code may specify a reference picture listconstruction process. The array RefPicTemporalListX may indicate areference picture list (e.g., intermediate reference picture list)comprising temporal reference pictures (e.g., only temporal referencepictures). The construction process for RefPicTemporalListX may bebackward compatible with single layer coding system and may be doneaccording to a single layer video coding implementation (e.g., HEVC orH.264/AVC). The array RefPicInterLayerListX may be a reference picturelist (e.g., intermediate reference picture list) comprising inter-layerreference pictures (e.g., only inter-layer reference pictures). Theoutput array RefPicListX may be a reference picture list (e.g., thefinal reference picture list) that comprises temporal and inter-layerreference pictures.

  cIdx = 0   for( i = 0; i <= num_ref_pic_lx_active_minus1; i++)      RefPicListX[i] = RefPicTemporalListX[i];   for( i = 0; i <=num_mod_inter_layer_ref_pic_lx; i++) {     for(rIdx =num_ref_pic_lx_active_minus1+i ; rIdx > modified_listX_idx[i]; rIdx−−)      RefPicListX[rIdx] = RefPicListX[rIdx−1];    RefPicListX[modified_listX_idx[i]] =RefPicInterLayerListX[inter_layer_ref_pic_idx[i]];   }

FIG. 7 is a diagram illustrating an example of a reference picture listmodification process. FIG. 7 illustrates an example of one inter-layerreference picture (e.g., the collocated picture), but the signalingmethod may accommodate more than one inter layer reference picture.There may be no default placement of inter-layer reference pictureswithin the reference picture list of a slice. An indicator (e.g.,inter_layer_ref_pic_modification_flag) may not be sent. The indicatormay be inferred to be 1. Syntax elements (e.g., inter_layer_ref_pic_idxand modified_listX_idx) may be sent to indicate where to place aninter-layer reference picture in the reference picture list(s).

An enhancement layer picture/slice coded as a B picture/slice may havemore than one reference picture lists. For example, an enhancement layerpicture/slice coded as a B picture/slice may have two reference picturelists, list 0 and list 1. One or more inter-layer referencepictures/slices may be inserted into one or more reference picturelists, for example, for one or more of the input pictures/slices. One ormore inter-layer reference pictures/slices may be inserted into one listbut not both lists, for example, for one or more of the inputpictures/slices. One or more inter-layer reference pictures/slices maynot be inserted into either list. Similar implementations may beutilized for P picture/slices, which, for example, may have onereference picture list (e.g., list 0). For example, for one or more Pcoded pictures/slices, the inter-layer reference pictures/slices may beinserted into a list (e.g., list 0). For one or more P codedpictures/slices, one or more temporal reference pictures but notinter-layer reference pictures may be inserted into the list (e.g., list0). Inter-layer reference pictures may be inserted into both list 0 andlist 1 of B picture/slices at a first temporal layer, and inserted intolist 0 of B picture/slices at higher temporal layers of a hierarchicalcoding structure, for example, random access (RA). Inter-layer referencepictures may be inserted to list 0 of P picture/slices.

An indicator may be utilized to indicate whether or not one or moreinter-layer reference pictures may be inserted into one or morereference picture lists. For example, a variable (e.g., a one-bit flag)per reference picture list may be used to indicate whether or not one ormore inter-layer reference pictures may be inserted into a list. Thevariable may be applied to a sequence-level signaling set, apicture-level signaling set, and/or a slice-level signaling set, such asbut not limited to Video Parameter Set (VPS), Sequence Parameter Set(SPS), Picture Parameter Set (PPS), slice segment header, etc. Table 4illustrates an example where syntax may be added to a slice segmentheader to signal whether to insert an inter-layer reference picture in areference picture list.

Signaling indicating whether one or more inter-layer reference picturesmay be inserted into one or more lists may be independent from orcombined with reference picture repositioning, for example, as describedwith reference to FIG. 7. For example, it may be determined whether aninter-layer reference picture may be included in a reference picturelist, and repositioning may be performed on the inter-layer referencepicture. A reference picture may be inserted into a reference picturelist and/or a reference picture may be repositioned within the referencepicture list.

TABLE 4 Example of a Modified Slice Segment Header Syntaxslice_segment_header( ) { Descriptor . . .  if( tiles_enabled_flag || entropy_coding_sync_enabled_flag ) {   num_entry_point_offsets ue(v)  if( num_entry_point_offsets > 0 ) {    offset_len_minus1 ue(v)    for(i = 0; i < num_entry_point_offsets; i++ )     entry_point_offset[ i ]u(v)   }  }   if(layer_id > 0) {    if(slice_type = = P || slice_type == B )     inter_layer_ref_list0_enabled_flag u(1)    if(slice_type = = B)     inter_layer_ref_list1_enabled_flag u(1)   }  if(slice_header_extension_present_flag ) {   slice_header_extension_lengthue(v)   for( i = 0; i < slice_header_extension_length; i++)   slice_header_extension_data_byte[ i ] u(8)  }  byte_alignment( ) }

Referring to Table 4, an indicator (e.g.,inter_layer_ref_list0_enabled_flag) may indicate whether to insert aplurality of inter-layer reference pictures into a reference picture(e.g., list0). An indicator (e.g., inter_layer_ref_list1_enabled_flag)may indicate whether to insert an inter-layer reference picture inreference picture list1. An indicator (e.g., modified_listX_idx) may besignaled in a bitstream to specify the position of a reference picturelist where a reference picture (e.g., an inter-layer reference picture)may be positioned (e.g., inserted, repositioned, or the like). Theindicator may be a single indicator (e.g., modified_listX_idx). Theindicator may be signaled in a parameter set (e.g., video parameter set(VPS), sequence parameter set (SPS), picture parameter set (PPS), etc.)or a slice header for reference picture list construction.

FIG. 9B illustrates an example reference list construction with singleindicator signaling. A bitstream may include a temporal referencepicture list and/or an inter-layer reference picture set. The bitstreammay include an indicator (e.g., modified_listX_idx) that may indicate aposition (e.g., position two) within the reference picture list. Theindicator may be included in a slice header. A plurality of inter-layerreference pictures (e.g., POC_(0,3), POC_(0,1), POC_(0,4), andPOC_(0,5)) may be inserted into the reference picture list at theindicated position based on the indicator. The inter-layer referencepicture that included the indicator (e.g., POC_(0,3)) may be inserted atthe position (e.g., position two), while the rest of the plurality ofinter-layer reference pictures may be inserted at positions followingthe indicated position in the reference picture list (e.g., positionsthree, four, and five). The rest of the plurality of inter-layerreference pictures (e.g., POC_(0,1), POC_(0,4), and POC_(0,5)) may beinserted into the reference picture list even though an indicator(s)indicating that the rest of the plurality of inter-layer referencepictures may not be received.

The reference pictures (e.g., temporal reference pictures POC_(1,4) andPOC_(1,5)) that were previously positioned in the reference picture listat those positions may be shifted, for example, according to theindication. For example, the temporal reference pictures POC_(1,4) andPOC_(1,5) may be repositioned to positions six and seven in thereference picture list. The reference pictures (e.g., temporal referencepictures POC_(1,4) and POC_(1,5)) that were previously positioned in thereference picture list at those positions may be shifted withoutreceiving an indication to shift the reference pictures. The shift maybe performed based on the indication to insert the inter-layer referencepicture into the reference picture list. By inserting one or moreinter-layer reference pictures, the reference picture list may increasein size (e.g., from four to eight).

The following may be conducted to construct a reference picture list(e.g., RefPicListX) with a single indicator (e.g., modified_listX_idx):

cIdx = 0 for( rIdx=0; rIdx <= num_ref_idx_lX_active_minus1; rIdx++) {   if ( rIdx == modified_listX_idx ) {     i = 0;     j =sizeof(RefPicInterLayerListX);   // number of inter-layer     referencepictures   while ((j) && (rIdx <= num_ref_idx_lX_active_minus1))     {     RefPicListX[rIdx++] = RefPicInterLayerListX [i++];      j −−;     }   }    else {      RefPicListX[rIdx++] = RefPicTempListX[cIdx++];    }}

Inter-layer reference pictures may be assumed to be placed at the end ofthe temporal list (e.g., RefPicTemporalListX) to form a referencepicture list (e.g., RefPicListX), for example, as provided in amultiview and/or a scalable video coding standard (e.g., H.264/AVC orHEVC). If the inter-layer reference pictures are not placed at the endof the reference picture list, then additional processing may beperformed when decoding the bitstream.

Intelligent reference picture reordering techniques based on multi-passencoding may be utilized to improve video coding efficiency. Multi-passencoding may be a video encoding technique where a video signal may beencoded by more than one pass. For example, in a first encoding pass,the video signal may be analyzed and encoding statistics may bepreserved. The statistics may be used in subsequent passes todynamically adjust encoding parameters for improved quality. Multi-passencoding may be performed on slice level, picture level, GOP level,and/or sequence level. Although the reference picture list reorderingtechniques described herein utilize two encoding passes as an example,additional passes (e.g., more than two encoding passes) may be applied.

A first-pass may encode a picture or slice using an initial referencepicture list for mixed temporal and inter-layer prediction. Thefirst-pass may collect relevant encoding statistics. In a second pass,the collected statistics may be used to reorder the temporal andinter-layer reference pictures. The second pass may use the reorderedreference picture lists to perform encoding. To reduce computationalcomplexity, the encoder may retain and compare the generated bit-streamsand reconstructed video signals of the first-pass and the second-pass.The encoder may output the bit-stream of the pass that minimizes apre-defined quality criterion, for example, the Lagrangianrate-distortion (RD) cost. FIG. 8 is a flowchart that illustrates anexample of an encoder using a reference picture reordering process.Although FIG. 8 provides an example using two encoding passes, referencepicture reordering methods may be applied to more than two passes byiterating the same reordering process for each additional pass.

To modify the initialized reference lists in the second-pass, varioustypes of video statistics may be used to determine the position index ofan reference picture in the reordered reference picture lists. Forexample, reference picture list reordering may be performed based onreference usage. An encoder may count the usage of a reference picture(e.g., each reference picture) in the reference picture list during afirst pass. For example, the usage may be collected based on the numberof smallest prediction units (SPU) in the currently encoded pictureand/or slice that refer to a given reference picture. In a second pass,the encoder may reorder the reference pictures in reference picturelists based on the frequency of their usage.

If multiple reference pictures are present in the one or more referencepicture lists, for an encoded block (e.g., a block whose one or morereference picture indices are sent), sending reference indices of largevalues may incur more overhead bits. The more frequently used referencepictures may be placed in the lower indices of one or more referencepicture lists, which may improve coding efficiency. This may reduce theoverhead associated with sending reference indices at block level.During a first encoding pass, the encoder may encode a picture or aslice while collecting statistics of reference picture usage. The usagemay be collected based on different metrics. For example, metrics mayinclude, but are not limited to, the number of times a reference picture(e.g., each reference picture) is used and the number of video blocks(e.g., SPUs) that refer to one reference in the reference picture lists.The encoder may reorder the reference pictures in the reference picturelists according to their frequency of usage. For example, the encodermay reorder the reference pictures such that the reference pictures thatare used more frequently are assigned lower indices. The encoder mayperform one or more additional coding passes with the reorderedreference picture lists.

Reference picture list reordering may be based on motion-compensateddistortion. An encoder may calculate the motion-compensated distortionbetween the original video blocks (e.g., SPUs) and their predictionblocks based on motion-compensated prediction using the referencepictures in the reference picture list. The reference pictures may bereordered according to their motion-compensated distortions for thesecond-pass coding.

A reference picture may be selected in the process of motion estimationby a Lagrangian RD criterion that selects the reference picture whichminimizes the cost J_(pred), for example, as shown by:J _(pred) =D+λ _(pred) ×B _(pred)  Equation (1)

where D may be the motion-compensated distortion between one originalvideo block and its prediction block; B_(pred) may be the number ofcoded bits specifying motion vectors and reference picture index; andλ_(pred) may be a Lagrangain weighting factor for motion estimation.Different metrics may be applied to derive motion-compensateddistortion, D, such as, but not limited to, sum of square error (SSE),sum of absolute difference (SAD), and sum of absolute transformeddifferences (SATD). The reference pictures located at the beginning ofthe initialized reference picture list may have a higher priority thanother reference pictures to be selected for inter-prediction (e.g., asillustrated by Equation (1)), which may be due to smaller referenceposition indices. Reference pictures, which may be more correlated withthe current picture but associated with high reference picture indices,may fail to be chosen by the encoder due to the heavy RD weighting partin Equation (1) imposed by the bits used to encode their large indicesof the initialized reference picture list.

Indices may be assigned to reference pictures based on theirmotion-compensated distortion to a current picture and/or slice. Forexample, as shown in Equation (1), the encoder may collect in afirst-pass the motion-compensated distortion of a coding block (e.g.,SPU) when it is predicted from different references in the initializedreference picture list. The encoder may sum up the corresponding totaldistortion for a reference picture in reference picture list. In thesecond-pass, the encoder may modify the indices in a reference picturelist based on the corresponding total distortion. Accordingly, if areference picture presents a smaller motion-compensated distortion tothe current picture after motion compensation, it may be given a smallerindex in the reordered reference picture list because it is more usefulfor inter-prediction. Implementations based on motion-compensateddistortion may avoid the interference of the initialized referencepicture indices, for example, as described herein. Since one or moremotion-compensated distortions may be available during the motionestimation and compensation processes of the first-pass, implementationsbased on motion-compensated distortion may incur less computationcomplexity.

Various video distortion metrics may be applied to distortion-basedreference picture reordering implementations, which may result indifferent coding performances. For example, SAD and SATD may be used formotion estimation. Although SATD may be more computationally complicatedthan SAD by taking an additional frequency transform (e.g., a Hadamardtransform) on the difference between the original video block and itsprediction block, SATD may more accurately predict video distortion fromthe viewpoint of both objective and subjective metrics. SATD may be usedas the distortion metric for reference picture list reordering in theimplementations described herein to pursue the largest coding gain.

Although the reference picture reordering implementations describedherein may reorder reference pictures in list0 and list1, they may beequally applied to modify any other reference picture lists, forexample, such as but not limited to the combined reference picture listin HEVC.

Signaling of a reference picture list (e.g., reordering and/or insertionof a reference picture) may be performed after a reference picturereordering technique based on multi-pass encoding, for example, asdescribed herein. Constraints may be added to a reference picturereordering technique based on multi-pass encoding, for example, toensure that a sub-set of reference pictures are kept continuous in thereference picture list. For example, inter-layer reference pictures maybe kept continuous (e.g., in continuous positions) within a referencepicture list after a reference picture reordering technique based onmulti-pass encoding is performed.

FIG. 10A is a diagram of an example communications system 1000 in whichone or more disclosed embodiments may be implemented. The communicationssystem 1000 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 1000 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems1000 may employ one or more channel access methods, such as codedivision multiple access (CDMA), time division multiple access (TDMA),frequency division multiple access (FDMA), orthogonal FDMA (OFDMA),single-carrier FDMA (SC-FDMA), and the like.

As shown in FIG. 10A, the communications system 1000 may includewireless transmit/receive units (WTRUs) 1002 a, 1002 b, 1002 c, 1002 d,a radio access network (RAN) 1003/1004/1005, a core network1006/1007/1009, a public switched telephone network (PSTN) 1008, theInternet 1010, and other networks 1012, though it will be appreciatedthat the disclosed embodiments contemplate any number of WTRUs, basestations, networks, and/or network elements. Each of the WTRUs 1002 a,1002 b, 1002 c, 1002 d may be any type of device configured to operateand/or communicate in a wireless environment. By way of example, theWTRUs 1002 a, 1002 b, 1002 c, 1002 d may be configured to transmitand/or receive wireless signals and may include user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a smartphone, a laptop, anetbook, a personal computer, a wireless sensor, consumer electronics,and the like.

The communications systems 1000 may also include a base station 1014 aand a base station 1014 b. Each of the base stations 1014 a, 1014 b maybe any type of device configured to wirelessly interface with at leastone of the WTRUs 1002 a, 1002 b, 1002 c, 1002 d to facilitate access toone or more communication networks, such as the core network1006/1007/1009, the Internet 1010, and/or the networks 1012. By way ofexample, the base stations 1014 a, 1014 b may be a base transceiverstation (BTS), a Node-B, an eNode B, a Home Node B, a Home eNode B, asite controller, an access point (AP), a wireless router, and the like.While the base stations 1014 a, 1014 b are each depicted as a singleelement, it will be appreciated that the base stations 1014 a, 1014 bmay include any number of interconnected base stations and/or networkelements.

The base station 1014 a may be part of the RAN 1003/1004/1005, which mayalso include other base stations and/or network elements (not shown),such as a base station controller (BSC), a radio network controller(RNC), relay nodes, etc. The base station 1014 a and/or the base station1014 b may be configured to transmit and/or receive wireless signalswithin a particular geographic region, which may be referred to as acell (not shown). The cell may further be divided into cell sectors. Forexample, the cell associated with the base station 1014 a may be dividedinto three sectors. Thus, in one embodiment, the base station 1014 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 1014 a may employ multiple-inputmultiple output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 1014 a, 1014 b may communicate with one or more of theWTRUs 1002 a, 1002 b, 1002 c, 1002 d over an air interface1015/1016/1017, which may be any suitable wireless communication link(e.g., radio frequency (RF), microwave, infrared (IR), ultraviolet (UV),visible light, etc.). The air interface 1015/1016/1017 may beestablished using any suitable radio access technology (RAT).

More specifically, as noted above, the communications system 1000 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 1014 a in the RAN 1003/1004/1005 and the WTRUs1002 a, 1002 b, 1002 c may implement a radio technology such asUniversal Mobile Telecommunications System (UMTS) Terrestrial RadioAccess (UTRA), which may establish the air interface 1015/1016/1017using wideband CDMA (WCDMA). WCDMA may include communication protocolssuch as High-Speed Packet Access (HSPA) and/or Evolved HSPA (HSPA+).HSPA may include High-Speed Downlink Packet Access (HSDPA) and/orHigh-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 1014 a and the WTRUs 1002 a,1002 b, 1002 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface1015/1016/1017 using Long Term Evolution (LTE) and/or LTE-Advanced(LTE-A).

In other embodiments, the base station 1014 a and the WTRUs 1002 a, 1002b, 1002 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1×, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 1014 b in FIG. 10A may be a wireless router, Home NodeB, Home eNode B, or access point, for example, and may utilize anysuitable RAT for facilitating wireless connectivity in a localized area,such as a place of business, a home, a vehicle, a campus, and the like.In one embodiment, the base station 1014 b and the WTRUs 1002 c, 1002 dmay implement a radio technology such as IEEE 802.11 to establish awireless local area network (WLAN). In another embodiment, the basestation 1014 b and the WTRUs 1002 c, 1002 d may implement a radiotechnology such as IEEE 802.15 to establish a wireless personal areanetwork (WPAN). In yet another embodiment, the base station 1014 b andthe WTRUs 1002 c, 1002 d may utilize a cellular-based RAT (e.g., WCDMA,CDMA2000, GSM, LTE, LTE-A, etc.) to establish a picocell or femtocell.As shown in FIG. 10A, the base station 1014 b may have a directconnection to the Internet 1010. Thus, the base station 1014 b may notbe required to access the Internet 1010 via the core network1006/1007/1009.

The RAN 1003/1004/1005 may be in communication with the core network1006/1007/1009, which may be any type of network configured to providevoice, data, applications, and/or voice over internet protocol (VoIP)services to one or more of the WTRUs 1002 a, 1002 b, 1002 c, 1002 d. Forexample, the core network 1006/1007/1009 may provide call control,billing services, mobile location-based services, pre-paid calling,Internet connectivity, video distribution, etc., and/or performhigh-level security functions, such as user authentication. Although notshown in FIG. 10A, it will be appreciated that the RAN 1003/1004/1005and/or the core network 1006/1007/1009 may be in direct or indirectcommunication with other RANs that employ the same RAT as the RAN1003/1004/1005 or a different RAT. For example, in addition to beingconnected to the RAN 1003/1004/1005, which may be utilizing an E-UTRAradio technology, the core network 1006/1007/1009 may also be incommunication with another RAN (not shown) employing a GSM radiotechnology.

The core network 1006/1007/1009 may also serve as a gateway for theWTRUs 1002 a, 1002 b, 1002 c, 1002 d to access the PSTN 1008, theInternet 1010, and/or other networks 1012. The PSTN 1008 may includecircuit-switched telephone networks that provide plain old telephoneservice (POTS). The Internet 1010 may include a global system ofinterconnected computer networks and devices that use commoncommunication protocols, such as the transmission control protocol(TCP), user datagram protocol (UDP) and the internet protocol (IP) inthe TCP/IP internet protocol suite. The networks 1012 may include wiredor wireless communications networks owned and/or operated by otherservice providers. For example, the networks 1012 may include anothercore network connected to one or more RANs, which may employ the sameRAT as the RAN 1003/1004/1005 or a different RAT.

Some or all of the WTRUs 1002 a, 1002 b, 1002 c, 1002 d in thecommunications system 1000 may include multi-mode capabilities, i.e.,the WTRUs 1002 a, 1002 b, 1002 c, 1002 d may include multipletransceivers for communicating with different wireless networks overdifferent wireless links. For example, the WTRU 1002 c shown in FIG. 10Amay be configured to communicate with the base station 1014 a, which mayemploy a cellular-based radio technology, and with the base station 1014b, which may employ an IEEE 802 radio technology.

FIG. 10B is a system diagram of an example WTRU 1002. As shown in FIG.10B, the WTRU 1002 may include a processor 1018, a transceiver 1020, atransmit/receive element 1022, a speaker/microphone 1024, a keypad 1026,a display/touchpad 1028, non-removable memory 1030, removable memory1032, a power source 1034, a global positioning system (GPS) chipset1036, and other peripherals 1038. It will be appreciated that the WTRU1002 may include any sub-combination of the foregoing elements whileremaining consistent with an embodiment. It is noted that thecomponents, functions, and features described with respect to the WTRU1002 may also be similarly implemented in a base station.

The processor 1018 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 1018 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 1002 to operate in a wirelessenvironment. The processor 1018 may be coupled to the transceiver 1020,which may be coupled to the transmit/receive element 1022. While FIG.10B depicts the processor 1018 and the transceiver 1020 as separatecomponents, it will be appreciated that the processor 1018 and thetransceiver 1020 may be integrated together in an electronic package orchip.

The transmit/receive element 1022 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 1014a) over the air interface 1015/1016/1017. For example, in oneembodiment, the transmit/receive element 1022 may be an antennaconfigured to transmit and/or receive RF signals. In another embodiment,the transmit/receive element 1022 may be an emitter/detector configuredto transmit and/or receive IR, UV, or visible light signals, forexample. In yet another embodiment, the transmit/receive element 1022may be configured to transmit and receive both RF and light signals. Itwill be appreciated that the transmit/receive element 1022 may beconfigured to transmit and/or receive any combination of wirelesssignals.

In addition, although the transmit/receive element 1022 is depicted inFIG. 10B as a single element, the WTRU 1002 may include any number oftransmit/receive elements 1022. More specifically, the WTRU 1002 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 1002 mayinclude two or more transmit/receive elements 1022 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 1015/1016/1017.

The transceiver 1020 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 1022 and to demodulatethe signals that are received by the transmit/receive element 1022. Asnoted above, the WTRU 1002 may have multi-mode capabilities. Thus, thetransceiver 1020 may include multiple transceivers for enabling the WTRU1002 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 1018 of the WTRU 1002 may be coupled to, and may receiveuser input data from, the speaker/microphone 1024, the keypad 1026,and/or the display/touchpad 1028 (e.g., a liquid crystal display (LCD)display unit or organic light-emitting diode (OLED) display unit). Theprocessor 1018 may also output user data to the speaker/microphone 1024,the keypad 1026, and/or the display/touchpad 1028. In addition, theprocessor 1018 may access information from, and store data in, any typeof suitable memory, such as the non-removable memory 1030 and/or theremovable memory 1032. The non-removable memory 1030 may includerandom-access memory (RAM), read-only memory (ROM), a hard disk, or anyother type of memory storage device. The removable memory 1032 mayinclude a subscriber identity module (SIM) card, a memory stick, asecure digital (SD) memory card, and the like. In other embodiments, theprocessor 1018 may access information from, and store data in, memorythat is not physically located on the WTRU 1002, such as on a server ora home computer (not shown).

The processor 1018 may receive power from the power source 1034, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 1002. The power source 1034 may be any suitabledevice for powering the WTRU 1002. For example, the power source 1034may include one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 1018 may also be coupled to the GPS chipset 1036, whichmay be configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 1002. In additionto, or in lieu of, the information from the GPS chipset 1036, the WTRU1002 may receive location information over the air interface1015/1016/1017 from a base station (e.g., base stations 1014 a, 1014 b)and/or determine its location based on the timing of the signals beingreceived from two or more nearby base stations. It will be appreciatedthat the WTRU 1002 may acquire location information by way of anysuitable location-determination method while remaining consistent withan embodiment.

The processor 1018 may further be coupled to other peripherals 1038,which may include one or more software and/or hardware modules thatprovide additional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 1038 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 10C is a system diagram of the RAN 1003 and the core network 1006according to an embodiment. As noted above, the RAN 1003 may employ aUTRA radio technology to communicate with the WTRUs 1002 a, 1002 b, 1002c over the air interface 1015. The RAN 1003 may also be in communicationwith the core network 1006. As shown in FIG. 10C, the RAN 1003 mayinclude Node-Bs 1040 a, 1040 b, 1040 c, which may each include one ormore transceivers for communicating with the WTRUs 1002 a, 1002 b, 1002c over the air interface 1015. The Node-Bs 1040 a, 1040 b, 1040 c mayeach be associated with a particular cell (not shown) within the RAN1003. The RAN 1003 may also include RNCs 1042 a, 1042 b. It will beappreciated that the RAN 1003 may include any number of Node-Bs and RNCswhile remaining consistent with an embodiment.

As shown in FIG. 10C, the Node-Bs 1040 a, 1040 b may be in communicationwith the RNC 1042 a. Additionally, the Node-B 1040 c may be incommunication with the RNC 1042 b. The Node-Bs 1040 a, 1040 b, 1040 cmay communicate with the respective RNCs 1042 a, 1042 b via an Iubinterface. The RNCs 1042 a, 1042 b may be in communication with oneanother via an Iur interface. Each of the RNCs 1042 a, 1042 b may beconfigured to control the respective Node-Bs 1040 a, 1040 b, 1040 c towhich it is connected. In addition, each of the RNCs 1042 a, 1042 b maybe configured to carry out or support other functionality, such as outerloop power control, load control, admission control, packet scheduling,handover control, macrodiversity, security functions, data encryption,and the like.

The core network 1006 shown in FIG. 10C may include a media gateway(MGW) 1044, a mobile switching center (MSC) 1046, a serving GPRS supportnode (SGSN) 1048, and/or a gateway GPRS support node (GGSN) 1050. Whileeach of the foregoing elements are depicted as part of the core network1006, it will be appreciated that any one of these elements may be ownedand/or operated by an entity other than the core network operator.

The RNC 1042 a in the RAN 1003 may be connected to the MSC 1046 in thecore network 1006 via an IuCS interface. The MSC 1046 may be connectedto the MGW 1044. The MSC 1046 and the MGW 1044 may provide the WTRUs1002 a, 1002 b, 1002 c with access to circuit-switched networks, such asthe PSTN 1008, to facilitate communications between the WTRUs 1002 a,1002 b, 1002 c and traditional land-line communications devices.

The RNC 1042 a in the RAN 1003 may also be connected to the SGSN 1048 inthe core network 1006 via an IuPS interface. The SGSN 1048 may beconnected to the GGSN 1050. The SGSN 1048 and the GGSN 1050 may providethe WTRUs 1002 a, 1002 b, 1002 c with access to packet-switchednetworks, such as the Internet 1010, to facilitate communicationsbetween and the WTRUs 1002 a, 1002 b, 1002 c and IP-enabled devices.

As noted above, the core network 1006 may also be connected to thenetworks 1012, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

FIG. 10D is a system diagram of the RAN 1004 and the core network 1007according to an embodiment. As noted above, the RAN 1004 may employ anE-UTRA radio technology to communicate with the WTRUs 1002 a, 1002 b,1002 c over the air interface 1016. The RAN 1004 may also be incommunication with the core network 1007.

The RAN 1004 may include eNode-Bs 1060 a, 1060 b, 1060 c, though it willbe appreciated that the RAN 1004 may include any number of eNode-Bswhile remaining consistent with an embodiment. The eNode-Bs 1060 a, 1060b, 1060 c may each include one or more transceivers for communicatingwith the WTRUs 1002 a, 1002 b, 1002 c over the air interface 1016. Inone embodiment, the eNode-Bs 1060 a, 1060 b, 1060 c may implement MIMOtechnology. Thus, the eNode-B 1060 a, for example, may use multipleantennas to transmit wireless signals to, and receive wireless signalsfrom, the WTRU 1002 a.

Each of the eNode-Bs 1060 a, 1060 b, 1060 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 10D, theeNode-Bs 1060 a, 1060 b, 1060 c may communicate with one another over anX2 interface.

The core network 1007 shown in FIG. 10D may include a mobilitymanagement gateway (MME) 1062, a serving gateway 1064, and a packet datanetwork (PDN) gateway 1066. While each of the foregoing elements aredepicted as part of the core network 1007, it will be appreciated thatany one of these elements may be owned and/or operated by an entityother than the core network operator.

The MME 1062 may be connected to each of the eNode-Bs 1060 a, 1060 b,1060 c in the RAN 1004 via an S1 interface and may serve as a controlnode. For example, the MME 1062 may be responsible for authenticatingusers of the WTRUs 1002 a, 1002 b, 1002 c, beareractivation/deactivation, selecting a particular serving gateway duringan initial attach of the WTRUs 1002 a, 1002 b, 1002 c, and the like. TheMME 1062 may also provide a control plane function for switching betweenthe RAN 1004 and other RANs (not shown) that employ other radiotechnologies, such as GSM or WCDMA.

The serving gateway 1064 may be connected to each of the eNode Bs 1060a, 1060 b, 1060 c in the RAN 1004 via the S1 interface. The servinggateway 1064 may generally route and forward user data packets to/fromthe WTRUs 1002 a, 1002 b, 1002 c. The serving gateway 1064 may alsoperform other functions, such as anchoring user planes duringinter-eNode B handovers, triggering paging when downlink data isavailable for the WTRUs 1002 a, 1002 b, 1002 c, managing and storingcontexts of the WTRUs 1002 a, 1002 b, 1002 c, and the like.

The serving gateway 1064 may also be connected to the PDN gateway 1066,which may provide the WTRUs 1002 a, 1002 b, 1002 c with access topacket-switched networks, such as the Internet 1010, to facilitatecommunications between the WTRUs 1002 a, 1002 b, 1002 c and IP-enableddevices.

The core network 1007 may facilitate communications with other networks.For example, the core network 1007 may provide the WTRUs 1002 a, 1002 b,1002 c with access to circuit-switched networks, such as the PSTN 1008,to facilitate communications between the WTRUs 1002 a, 1002 b, 1002 cand traditional land-line communications devices. For example, the corenetwork 1007 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 1007 and the PSTN 1008. In addition, the corenetwork 1007 may provide the WTRUs 1002 a, 1002 b, 1002 c with access tothe networks 1012, which may include other wired or wireless networksthat are owned and/or operated by other service providers.

FIG. 10E is a system diagram of the RAN 1005 and the core network 1009according to an embodiment. The RAN 1005 may be an access servicenetwork (ASN) that employs IEEE 802.16 radio technology to communicatewith the WTRUs 1002 a, 1002 b, 1002 c over the air interface 1016. Aswill be further discussed below, the communication links between thedifferent functional entities of the WTRUs 1002 a, 1002 b, 1002 c, theRAN 1005, and the core network 1009 may be defined as reference points.

As shown in FIG. 10E, the RAN 1005 may include base stations 1080 a,1080 b, 1080 c, and an ASN gateway 1082, though it will be appreciatedthat the RAN 1005 may include any number of base stations and ASNgateways while remaining consistent with an embodiment. The basestations 1080 a, 1080 b, 1080 c may each be associated with a particularcell (not shown) in the RAN 1005 and may each include one or moretransceivers for communicating with the WTRUs 1002 a, 1002 b, 1002 cover the air interface 1017. In one embodiment, the base stations 1080a, 1080 b, 1080 c may implement MIMO technology. Thus, the base station1080 a, for example, may use multiple antennas to transmit wirelesssignals to, and receive wireless signals from, the WTRU 1002 a. The basestations 1080 a, 1080 b, 1080 c may also provide mobility managementfunctions, such as handoff triggering, tunnel establishment, radioresource management, traffic classification, quality of service (QoS)policy enforcement, and the like. The ASN gateway 1082 may serve as atraffic aggregation point and may be responsible for paging, caching ofsubscriber profiles, routing to the core network 1009, and the like.

The air interface 1017 between the WTRUs 1002 a, 1002 b, 1002 c and theRAN 1005 may be defined as an R1 reference point that implements theIEEE 802.16 specification. In addition, each of the WTRUs 1002 a, 1002b, 1002 c may establish a logical interface (not shown) with the corenetwork 1009. The logical interface between the WTRUs 1002 a, 1002 b,1002 c and the core network 1009 may be defined as an R2 referencepoint, which may be used for authentication, authorization, IP hostconfiguration management, and/or mobility management.

The communication link between each of the base stations 1080 a, 1080 b,1080 c may be defined as an R8 reference point that includes protocolsfor facilitating WTRU handovers and the transfer of data between basestations. The communication link between the base stations 1080 a, 1080b, 1080 c and the ASN gateway 1082 may be defined as an R6 referencepoint. The R6 reference point may include protocols for facilitatingmobility management based on mobility events associated with each of theWTRUs 1002 a, 1002 b, 1002 c.

As shown in FIG. 10E, the RAN 1005 may be connected to the core network1009. The communication link between the RAN 1005 and the core network1009 may defined as an R3 reference point that includes protocols forfacilitating data transfer and mobility management capabilities, forexample. The core network 1009 may include a mobile IP home agent(MIP-HA) 1084, an authentication, authorization, accounting (AAA) server1086, and a gateway 1088. While each of the foregoing elements aredepicted as part of the core network 1009, it will be appreciated thatany one of these elements may be owned and/or operated by an entityother than the core network operator.

The MIP-HA may be responsible for IP address management, and may enablethe WTRUs 1002 a, 1002 b, 1002 c to roam between different ASNs and/ordifferent core networks. The MIP-HA 1084 may provide the WTRUs 1002 a,1002 b, 1002 c with access to packet-switched networks, such as theInternet 1010, to facilitate communications between the WTRUs 1002 a,1002 b, 1002 c and IP-enabled devices. The AAA server 1086 may beresponsible for user authentication and for supporting user services.The gateway 1088 may facilitate interworking with other networks. Forexample, the gateway 1088 may provide the WTRUs 1002 a, 1002 b, 1002 cwith access to circuit-switched networks, such as the PSTN 1008, tofacilitate communications between the WTRUs 1002 a, 1002 b, 1002 c andtraditional land-line communications devices. In addition, the gateway1088 may provide the WTRUs 1002 a, 1002 b, 1002 c with access to thenetworks 1012, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

Although not shown in FIG. 10E, it will be appreciated that the RAN 1005may be connected to other ASNs and the core network 1009 may beconnected to other core networks. The communication link between the RAN1005 the other ASNs may be defined as an R4 reference point, which mayinclude protocols for coordinating the mobility of the WTRUs 1002 a,1002 b, 1002 c between the RAN 1005 and the other ASNs. Thecommunication link between the core network 1009 and the other corenetworks may be defined as an R5 reference, which may include protocolsfor facilitating interworking between home core networks and visitedcore networks.

The processes described above may be implemented in a computer program,software, or firmware incorporated in a computer-readable medium forexecution by a computer or processor. Examples of computer-readablemedia include electronic signals (transmitted over wired or wirelessconnections) and computer-readable storage media. Examples ofcomputer-readable storage media include, but are not limited to, a readonly memory (ROM), a random access memory (RAM), a register, cachememory, semiconductor memory devices, magnetic media such as internalhard disks and removable disks, magneto-optical media, and optical mediasuch as CD-ROM disks, and digital versatile disks (DVDs). A processor inassociation with software may be used to implement a radio frequencytransceiver for use in a WTRU, UE, terminal, base station, RNC, or anyhost computer.

What is claimed:
 1. A method comprising: receiving a video signal;determining encoding statistics related to the video signal during afirst encoding pass using one or more reference picture lists;reordering, prior to a second encoding pass, pictures in the one or morereference picture lists based on the encoding statistics determined fromthe first encoding pass, wherein the encoding statistics comprisemotion-compensated distortion information such that the pictures withsmaller motion-compensated distortion are assigned lower indices in theone or more reference picture lists; and encoding the video signalduring the second encoding pass using the one or more reorderedreference picture lists.
 2. The method of claim 1, further comprising:encoding the video signal during the first encoding pass using the oneor more reference picture lists; comparing a video bitstream generatedduring the first encoding pass to a video bitstream generated during thesecond encoding pass; and sending the video bitstream that ischaracterized by a lower Lagrangian rate-distortion (RD) cost.
 3. Themethod of claim 1, wherein the encoding statistics further comprisereference usage information, and wherein the pictures in the one or morereference picture lists are reordered such that the pictures with higherusage frequency are assigned lower indices.
 4. The method of claim 3,wherein the reference usage information is based on a number of smallestprediction units (SPUs) in an encoded picture or slice that refer to areference picture.
 5. The method of claim 1, wherein themotion-compensated distortion information is determined based on one ormore of sum of square error (SSE), sum of absolute difference (SAD), orsum of absolute transformed differences (SATD).
 6. The method of claim1, wherein the motion-compensated distortion information is determinedbased on motion-compensated distortion of smallest prediction units(SPUs) and prediction blocks of the pictures in the one or morereference picture lists.
 7. The method of claim 1, wherein one or moreof the first encoding pass and the second encoding pass comprise acomplete encoding pass of the video signal.
 8. The method of claim 1,wherein the pictures in the one or more reference picture lists that arereordered comprise at least one of temporal reference pictures orinter-layer reference pictures.
 9. The method of claim 1, furthercomprising: performing one or more encoding passes after the firstencoding pass and before the second encoding pass.
 10. An encodercomprising: a processor configured to: receive a video signal; determineencoding statistics related to the video signal during a first encodingpass using one or more reference picture lists; reorder, prior to asecond encoding pass, pictures in the one or more reference picturelists based on the encoding statistics determined from the firstencoding pass, wherein the encoding statistics comprisemotion-compensated distortion information such that the pictures withsmaller motion-compensated distortion are assigned lower indices in theone or more reference picture lists; and encode the video signal duringthe second encoding pass using the one or more reordered referencepicture lists.
 11. The encoder of claim 10, wherein the processor isfurther configured to: encode the video signal during the first encodingpass using the one or more reference picture lists; compare a videobitstream generated during the first encoding pass to a video bitstreamgenerated during the second encoding pass; and send the video bitstreamthat is characterized by a lower Lagrangian rate-distortion (RD) cost.12. The encoder of claim 10, wherein the encoding statistics furthercomprise reference usage information, and wherein the pictures in theone or more reference picture lists are reordered such that the pictureswith higher usage frequency are assigned lower indices.
 13. The encoderof claim 12, wherein the reference usage information is based on anumber of smallest prediction units (SPUs) in an encoded picture orslice that refer to a reference picture.
 14. The encoder of claim 10,wherein the motion-compensated distortion information is determinedbased on one or more of sum of square error (SSE), sum of absolutedifference (SAD), or sum of absolute transformed differences (SATD). 15.The encoder of claim 10, wherein the motion-compensated distortioninformation is determined based on motion-compensated distortion ofsmallest prediction units (SPUs) and prediction blocks of the picturesin the one or more reference picture lists.
 16. The encoder of claim 10,wherein one or more of the first encoding pass and the second encodingpass comprise a complete encoding pass of the video signal.
 17. Theencoder of claim 10, wherein the pictures in the one or more referencepicture lists that are reordered comprise at least one of temporalreference pictures or inter-layer reference pictures.
 18. The encoder ofclaim 10, wherein the encoder is further configured to perform one ormore encoding passes after the first encoding pass and before the secondencoding pass.